This report includes work arising from the following clinical protocol: NCT01434368. We have recruited approximately half of the proposed sample, and several of the children in the original 12-13 year-old cohort have completed the study through to their 18th birthday. To date we have studied 44 prepubertal children (27 boys and 17 girls), who are clearly identified as pre-gonadarchal by Tanner staging and bone age. These children also have plasma levels of estradiol and testosterone at the lower limits of detectability, and have a bimodal distribution of plasma adrenal androgen levels (e.g., DHEA-S, androstenedione) indicating both boys and girls are in either early or later stages of adrenarche. Preliminary neuroimaging findings point to sex-differences particularly in the medial prefrontal cortex (mPFC) and dorsolateral prefrontal cortex (DLPFC) in task-based paradigms, suggesting the presence of sex-differences prior to HPG axis (re)activation. Additionally, a connectome-wide association study (CWAS) in this prepubertal cohort identified sex-differences in resting state functional connectivity in the mPFC, a key region in the default-mode network (DMN). Using this mPFC region as a seed, post hoc analysis showed several clusters in the DMN and executive control network in which girls showed higher mPFC connectivity compared to boys, whereas no voxels showed higher connectivity in boys. These preliminary results suggest that there may be neural circuits that exhibit sex-differences prior to gonadarche and HPG axis activation. However, further analysis is needed to determine the factors contributing to sex-differences in this cohort. For example, in addition to possible independent sex-chromosomal effects, sex-differences could reflect greater exposures to adrenal androgens in those children who are in the later stages of adrenarche (which we can examine given the dichotomous distribution of adrenal androgen levels in this sample), or earlier sex steroid exposure secondary to the initial post-natal activation of the HPG axis which lasts 1-2 weeks in boys but up to two years in girls. Initial findings in our combined pre- and post-pubertal cohorts have identified several landmarks in reproductive aging that could have important implications for brain development. We have observed that post-menarcheal girls experience the commencement of fluctuations in estradiol secretion (presumably corresponding to the maturation of the normal architecture of the menstrual cycle). Given our findings of the importance of changes in ovarian steroid levels in RERMDs, these observations could signal an important physiologic event during which functional neurocircuitry undergoes reprogramming. We also have observed in both boys and girls that MRI-measured gonadal volumes increase across the pubertal transition. The individual inflection points in these volume curves identify another important reproductive event (i.e., the expansion of the gonad) that is a sentinel event indicating the initial activation of the GnRH pulse generator, gonadotropin secretion, and the beginning of gonadarche. As enrollment continues and our pre-pubertal children mature, the longitudinal analysis of the development of these sex-differences in brain activity (as well as developmental trajectories in brain that are not sexually dimorphic) could identify specific puberty-related reproductive or metabolic events relevant to normal brain development. These data also will serve as an important archival data set for studies of children at high risk for the development of behavioral disorders. In our larger cohort, preliminary results of multi-modal neuroimaging studies suggest that pubertal stage contributes to several measures of adolescent brain development and accounts for differences between adolescent and adult brain development that are both brain region- and sex-specific. First, , cognition-activated fMRI studies document differential working memory-related prefrontal-hippocampal functional connectivity in children (early puberty) and adolescents (late puberty). We found differential negative prefrontal-hippocampal functional coupling during working memory in children but not adolescents, which could indicate altered reciprocal coupling between an immature prefrontal cortex and the hippocampus, along with changes in the level of cooperativity between these brain regions accompanying prefrontal cortical development during adolescence. Second, in our investigations of the development of neural inhibition in which we use fMRI and the well-validated Stop Signal Task (a reliable metric of neural inhibition), in which a pre-potent go response must be inhibited, we have been unable to confirm previous reports of significant sex- or age-related differences in the brain that support this key cognitive function. We did demonstrate a main effect of task, consistent with the current literature in both adults and children, with activation in the inferior frontal gyrus bilaterally, the supplementary motor area, and the subthalamic nucleus. However, there were no significant effects of sex or pubertal status in these task-related brain regions, despite both sex and pubertal differences between behavioral measures, such as speed of go response and stop accuracy. We did observe a pubertal status-by-sex interaction in the right medial prefrontal cortex (BA10). Post-hoc testing showed there was a significant between-sex difference in the pre-pubertal cohort. This puberty group by sex interaction was found in the default mode network in a region where activity is reduced during non-referential goal related tasks; activation of this region also has shown to be related to social and emotional processes. Although preliminary, the sex difference in mPFC activation in the pre-pubertal children did not appear to be reflective of (or mediated by) differences in adrenal steroid levels either. Nonetheless, it may be possible that either adrenal steroid levels or organizational sex-differences in mPFC that emerge early in childhood are systematically altered during puberty. Indeed, we found that serum testosterone levels negatively correlated with mPFC activation in boys, possibly suggesting that when testosterone levels rise during puberty, they act to decrease mPFC activation, at least within the context of behavioral inhibition. Further investigation is needed to explore the correlates of the robust deactivation in pre-pubertal females. Finally, albeit preliminary, our findings in studies of emotional face-processing suggest that sex differences in socioemotional processing also are established prior to puberty and develop independent of gonadal sex-steroid secretion.